The present invention relates to a semiconductor device and, more particularly, to a semiconductor device in which an electric current is passed between a GaAs semiconductor layer and an InGaAlP semiconductor layer both having the same conductivity type.
InGaAlP materials now receive widespread attention as materials for light emitting devices in the short wavelength range, these materials having the largest energy gap among alloys of group III-V compound semiconductors, except for nitrides. In particular, those compositions capable of lattice matching with GaAs can offer satisfactory epitaxial growth, with fewer crystal defects, by the metal organic chemical vapor deposition method (hereinafter abbreviated as MOCVD method).
When manufacturing light emitting devices and electronic devices which contain InGaAlP materials as active parts, it is a frequent practice to bring such materials into contact with metals through GaAs, which is capable of lattice matching therewith for obtaining good ohmic contact (as described in "Applied Physics Letters," 48 (1986) p. 207, for example). However, the difference in energy gap between GaAs and InGaAlP materials is so large that discontinuous energy bands at the interface cause large notches or spikes that obstruct ohmic injection of electric current. In particular, a significant effect is more likely to be observed in p-type heterojunctions in which holes of low mobility serve as carriers.
One method for avoiding such an adverse effect is to dispose, between the GaAs layer and the InGaAlP layer, an InGaAlP layer having a lower Al composition ratio and an intermediate energy gap between those two layers, for the purpose of effecting the ohmic injection of electric current (as disclosed in Japanese Patent Laid-Open No. 62-200784 (1987), for example). However, the provision of such an InGaAlP intermediate energy gap layer does not necessarily offer an ohmic characteristic. As a result, the voltage drop at the interface can give rise to the problem of increasing the operating voltage of the device. When applied to semiconductor lasers, the resulting overheating creates high temperatures that impair the oscillation characteristics.
Thus, in an attempt to achieve ohmic contact through GaAs in a semiconductor device having its active part made of InGaAlP, good ohmic contact between InGaAlP and GaAs is not achieved and, hence, the device operating voltage increases and its thermal characteristic is degraded.